Effect of D-limonene on volatile fatty acidsproduction from anaerobic fermentation of waste activated sludge under pH regulation: Performance and mechanisms

D-limonene extracted from citrus peels possesses an inhibitory effect on methanogenic archaea. This study is aimed to bridge the research gap on the influence of D-limonene on volatile fatty acids (VFA) production from waste activated sludge (WAS) and to address the low VFA yield in standalone anaerobic fermentation of WAS. When the initial pH was not controlled, 1.00 g/g TSS D-limonene resulted in a VFA accumulation of 1175.45 f 101.36 mg/L (174.45 f 8.13 mgCOD/gVS). When the initial pH was controlled at 10 and the D-limonene concentration was 0.50 g/g TSS, the VFA accumulation reached 2707.44 f 183.65 mg/L (445.51 f 17.10 mgCOD/gVS). The pH-regulated D-limonene treatment enhanced solubilization and acidification, slightly inhibited hydrolysis, and significantly suppressed methanogenesis. D-limonene under alkaline conditions can increase the relative abundance of Clostridium_sensu_stricto, significantly enhancing acidification. Moreover, it markedly inhibited methanogenesis by particularly reducing the relative abundance of Methanothrix that was responsible for acetate consumption, thus favoring the accumulation of VFA. The research reveals the potential mechanism of pH regulation and D-limonene on anaerobic fermentation acid production, providing a theoretical basis for improving the acid production performance of the anaerobic fermentation of WAS.